1. Field of the Invention
This invention relates to a liquid crystal display, and more particularly to a liquid crystal display that is adapted to eliminate a vertical dimming phenomenon to improve a picture quality of a liquid crystal display panel.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls the light transmittance of a liquid crystal using an electric field to thereby display a picture. To achieve this, the LCD includes a liquid crystal display panel having liquid crystal cells arranged in a matrix, and a driving circuit for driving the liquid crystal display panel. In the liquid crystal display panel, gate lines and data lines cross each other and liquid crystal cells are provided at areas defined by such crossings of the gate lines with the data lines. The liquid crystal display panel is provided with transparent pixel electrodes and a common electrode for applying an electric field to the liquid crystal cells. Each pixel electrode is connected, via source and drain terminals of a thin film transistor (TFT) as a switching device, to any one of the data lines. The gate terminal of the TFT is connected to any one of the gate lines. Accordingly, the LCD controls the light transmittance by an electric field applied between the pixel electrode and the common electrode in response to a data voltage signal supplied for each liquid crystal cell, to display a picture.
Such an LCD uses inversion driving schemes, such as a frame inversion system, a line (or column) inversion system and a dot inversion system, in order to drive the liquid crystal cells in the liquid crystal display panel. The frame inversion system inverts the polarities of data signals applied to the liquid crystal cells in the liquid crystal display panel whenever a frame is changed. The line inversion system inverts the polarities of data signals applied to the liquid crystal cells in accordance with the lines (or columns) on the liquid crystal display panel. The dot inversion system allows data signals having polarities contrary to that of the data signals applied to the liquid crystal cells adjacent to each other in the vertical and horizontal directions to be applied to the liquid crystal cells in the liquid crystal display panel, and allows the polarities of data signals applied to all the liquid crystal cells in the liquid crystal display panel to be inverted every frame. The dot inversion system of such inversion driving schemes provides a picture having a better picture quality than that of the frame and line inversion systems.
Typically, such an LCD is driven with a frame frequency of 60 Hz. However, a system requiring low power consumption such as a notebook personal computer requires lowering the frame frequency from 50 to 30 Hz. Since the dot inversion system capable of providing an excellent picture quality described with reference to the above-mentioned inversion systems also generates a flicker as the frame frequency decreases a liquid crystal display panel driving method having a horizontal 2-dot inversion system as shown in FIG. 1A and FIG. 1B.
Referring to FIG. 1A and FIG. 1B, there are shown data polarity patterns applied to the liquid crystal cells in the liquid crystal display panel by the liquid crystal display panel driving method employing the horizontal 2-dot inversion system, which are divided into odd frames and even frames, respectively. With respect to the odd frames shown in FIG. 1A and the even frames shown in FIG. 1B, the data polarity pattern in the horizontal 2-dot inversion driving system is changed for each two liquid crystal cells, that is, for each two dots in the horizontal direction while being changed for each one dot in the vertical direction.
If such a horizontal 2-dot inversion system is used, then a phenomenon that DC voltage concentrates on the data line from almost a majority of the screen is eliminated because an identical period between colors of red (R), green (G) and blue (B) pixels and data polarity patterns becomes 12 dots in the horizontal direction, thereby reducing a flicker. However, a use of the horizontal 2-dot inversion system causes a brightness difference between odd-numbered lines and even-numbered lines of the data lines from the gray field, thereby generating a vertical dimming phenomenon.
The above-mentioned vertical dim caused by the horizontal 2-dot inversion driving is generated by a parasitic capacitance between the data line and the pixel electrode. This will be described in detail in conjunction with FIG. 2 below.
Referring to FIG. 2 and FIG. 3, a liquid crystal cell of the conventional LCD includes thin film transistors TFT provided at crossings between data lines DL and gate lines GL, and pixel electrodes PE connected to common electrodes Vcom and the thin film transistors TFT opposed to each other with having a liquid crystal therebetween.
The thin film transistor TFT has a gate electrode connected to the gate line GL, a source electrode connected to the data line DL and a drain electrode connected to the pixel electrode PE. The thin film transistor TFT is turned on when a scanning signal, that is, a gate high voltage Vgh from the gate line GL, is applied, to thereby supply the liquid crystal cell with a pixel signal from the data line DL. Further, the thin film transistor TFT is turned off when a gate low voltage Vgl from the gate line GL is applied, to thereby maintain a pixel signal charged in the liquid crystal cell.
The liquid crystal cell further includes a storage capacitor Cst in order to maintain the charged pixel electrode PE until the next pixel signal is charged. This storage capacitor Cst is provided between the pixel electrode PE and the pre-stage gate line GLn−1. Such a liquid crystal cell varies an alignment state of a liquid crystal cell in response to a pixel signal charged via the thin film transistor TFT for the purpose of controlling a light transmittance, thereby implementing a gray level.
As shown in FIG. 3, the liquid crystal cell can be equivalently expressed as a liquid crystal capacitor Clc, which generates a parasitic capacitance because it is adjacent to the data line DL and the pixel electrode PE. In this case, the parasitic capacitance includes a first parasitic capacitance Cdp between the left data line DLm−1 and the pixel electrode PE, and a second parasitic capacitance Cpd between the pixel electrode PE and the left data line DLm. Each of the first and second parasitic capacitance Cdp and Cpd causes a voltage variation in the data line and a capacitance coupling after a pixel signal is charged in the liquid crystal cell, thereby varying a voltage of the liquid crystal cell.
In other words, if the liquid crystal cell is driven by the horizontal 2-dot inversion system, then two pixels having the same data polarity pattern of the data signal differ by an average variation value ΔVp−dp of a pixel voltage A caused by a capacitance coupling of the first parasitic capacitance Cdp from an average variation value ΔVp−pd of a pixel voltage B caused by a capacitance coupling of the second parasitic capacitance Cpd, as shown in FIG. 4, with respect to a left pixel 10 and a light pixel 20.
In this case, an average variation value ΔVp−dp of a pixel voltage A caused by a capacitance coupling of the first parasitic capacitance Cdp or an average variation value ΔVp−pd of a pixel voltage B caused by a capacitance coupling of the second parasitic capacitance Cpd can be expressed by the following equation:ΔVp(Cdp or Cpd)={Cdp×ΔV(DLm−1)+Cpd×ΔV(DLm)}÷Ctotal  (1)
wherein Ctotal represents total capacitance of the pixel electrode PE.
Accordingly, at the right pixel 20, the values are cancelled with respect to each other like the dot inversion driving system. On the other hand, at the left pixel 10, an average variation value ΔVp−dp of a pixel voltage A caused by a capacitance coupling of the first parasitic capacitance Cdp is added to an average variation value ΔVp−pd of a pixel voltage B caused by a capacitance coupling of the second parasitic capacitance Cpd at the left pixel 10 without being cancelled. Thus, average variation values of the pixel voltages caused by the capacitance couplings at the left pixel 10 and at the light pixel 20 become different from each other.
Consequently, when the first parasitic capacitance Cdp is equal to the second parasitic capacitance Cpd, an effective value of the pixel voltage at the left pixel 10 becomes two times of ΔVp−dp in comparison with that at the light pixel 20. In this case, an effective value variation in the pixel voltage increases when the pixel voltage has a positive level, whereas an effective value variation in the pixel voltage decreases when the pixel voltage has a negative level. Accordingly, there occurs a phenomenon that the right pixel 20 becomes brighter than the left pixel 10 as shown in FIG. 5. As a result, the LCD employing the horizontal 2-dot inversion driving system generates a vertical dim (or vertical line) from the liquid crystal display panel.